1. Field of the Invention
The present invention relates to a toner for developing latent images into visible images in electrophotography. The present invention also relates to a developer, a developing device, a process cartridge, an image forming apparatus, and an image forming method, each using the above toner, and a method of manufacturing the above toner.
2. Description of the Background
Toners for forming visible images have been widely used in electrophotographic image forming apparatuses. Generally, a toner is comprised of resin particles containing a colorant.
There are various processes of manufacturing toner. It is well known that a toner manufactured in an aqueous medium advantageously has a small particle diameter and a narrow particle diameter distribution. On the other hand, toners are preferably comprised primarily of a polyester resin, which is well fixable on a recording medium.
In view of this situation, a method of manufacturing a toner in an aqueous medium, especially a toner having a small particle diameter and a narrow particle diameter distribution and comprised primarily of a polyester resin, has proposed.
Specifically, the proposed method includes the steps of dissolving or dispersing a polyester resin and a colorant in an organic solvent to prepare an oily liquid, dispersing the oily liquid in a surfactant-containing aqueous medium to form oil droplets, removing the organic solvent from the oil droplets to form toner particles, and washing and drying the toner particles. This method may be hereinafter referred to as the dissolution suspension method.
Disadvantageously, toner particles comprised primarily of a polyester resin (hereinafter “polyester-based toner particles”) are less chargeable than those comprised primarily of a styrene-acrylic resin. Such less-chargeable toner particles are disadvantageous for use in one-component developing systems, in which toner particles are charged by agitation or friction using a toner supply member (e.g., a roller), a toner bearing member (e.g., a roller), or a toner regulator (e.g., a blade), because they are inferior to two-component developing systems, in which toner particles are charged by agitation or friction with carrier particles such as iron powders, in terms of sufficient charging of toner particles.
In attempting to improve chargeability of polyester-based resin particles, one proposed approach involves providing a vinyl resin, generally having good chargeability, on the surfaces of such polyester-based resin particles.
For example, Japanese Patent Application Publication No. (hereinafter “JP-A”) 2006-206851 discloses polyester-based resin particles, the surfaces of which are covered with a vinyl resin containing a large amount of methacrylic-acid-originated carboxyl groups. However, such a large amount of carboxyl groups increases hygroscopicity of the surfaces of the particles, thereby degrading chargeability especially under high-temperature and high-humidity conditions.
JP-2006-285188-A discloses a core-shell toner, in which the shell is comprised of a vinyl copolymer resin and the core is comprised of a polyester-based resin. However, the disclosed content of styrene units in the vinyl copolymer resin is too small to improve chargeability of the toner, especially under high-temperature and high-humidity conditions.
The above core-shell toner also has a problem in its manufacture process, such that when the amount of polar groups (e.g., carboxyl group) in the vinyl copolymer resin is too small, raw materials cannot be normally granulated into toner particles of the desired shape and size.
JP-H05-333587-A discloses a toner manufactured through a wet granulation process in which a polyester resin and a colorant are formed into particles under wet environments, and the particles are aggregated, dried, and ground in the succeeding processes. To improve chargeability of the resulting toner particles, chargeable fine resin particles comprised of 80 parts of styrene and 20 parts of butyl acrylate are further fixed on the surfaces of the toner particles. However, the chargeable fine resin particles cannot uniformly exist over the whole surface of the toner particle because the toner particle does not have a homogenous smooth surface owing to the way that it is manufactured.